Sauté onions over medium heat in a pot large enough to accommodate the lentils and tomatoes. When onions begin to brown, add the ginger and spices, heating until fragrant.
Add the lentils and tomatoes to onions and mix well. Add the water and bring to a boil.
Cover and turn down to simmer until the lentils are soft. This will take about 20 minutes.
In the meantime, prepare the tempering ingredients. Put the oil, mustard seeds and coriander seeds in a small pan, and cook over medium high heat until the mustard seeds begin to pop.
Mix the toasted seeds and cilantro into the lentils. Season with salt and lime juice to suit your taste.